In order to increase spectral efficiency, one possible modulation scheme is to orthogonally modulate amplitude-shift keying (ASK) and differential phase-shift-keying (DPSK) or DQPSK signals. When an ASK signal has a dark-RZ or inversion return-to-zero (inv-RZ) shape, the combined phase signals such as DPSK or DQPSK are not degraded by the ASK signal, therefore the performance of the DPSK or DQPSK after transmission will be higher. See C. W. Chow, H. K. Tsang, “Optical label encoding and swapping using half-bit delayed dark RZ payload and DPSK label”, Opt. Express 13, 5325-5330 (2005); S. Pun, C. Chan, and L. Chen, “Demonstration of a novel optical transmitter for high-speed differential phase-shift-keying inverse-return-to-zero (DPSK/Inv-RZ) orthogonally modulated signals”, IEEE Photon. Technol. Lett., Vol. 17, No. 12, 2005: 2763-2765; T. Miyazaki, and F. Kubota, “Superposition of DQPSK over inverse-RZ for 3-bit/Symbol modulation-demodulation”, IEEE Photon. Technol. Lett., Vol. 16, No. 12, 2004: 2643-2645. Obviously, the key issue for this orthogonal modulation scheme is to generate a dark-RZ pulse. Chow, et al. employed a nonlinear optical loop mirror (NOLM) to generate dark RZ pulse. It is well known that it is difficult to have stable operations using a NOLM and that the NOLM has a high-power consumption. Pun, et al. demonstrated a novel scheme to generate a dark-RZ pulse, where the dark-RZ pulse was used for improving the spectral efficiency for orthogonal modulation format signals. This scheme uses data and inverted data to drive the dual-arm modulator, and then a DPSK signal with a dip can be generated. Since this scheme uses data and inverted data, two electrical amplifiers are necessary to amplifying the data and inverted data prior to driving the dual-arm modulator. This consequently increases costs. Miyazaki used cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA) to generate dark RZ pulse. In this scheme, it is necessary to generate a bright RZ pulse and also requires an SOA and optical filter for XGM. This configuration is complex and expensive.
There has been increasing interest in providing broadband wireless access services in emerging optical-wireless networks. In this regard, optical millimeter (mm)-wave generation is a key technique employed in such networks. Optical intensity modulators can be utilized to generate high-frequency mm waves. However, an electrical control circuit is needed to optimize the direct current (DC) bias added to the external intensity modulator in order to obtain high-quality mm-wave signals. An optical phase modulator does not require a DC bias and thus does not suffer from a DC bias-drifting problem. An external phase modulator followed by an optical notch filter could be utilized to produce optical mm waves. See, G. Qi, J. Yao, et al., “Optical generation and distribution of continuously tunable millimeter-wave signals using an optical phase modulator,” J. of Lightwave Technol., Vol. 23, No. 9, 2005: 2687-2695. However, due to the multiple sidebands created by a deep modulation index, the mm-wave generated by this methodology suffered from excessive fiber dispersion, and the transmission distance was thus limited to a few kilometers. Greater distances require dispersion compensation for such radio over fiber (ROF) systems, thereby increasing costs and reducing flexibility of dynamic systems where transmission distances to customer premises over access networks may not be fixed. Since WDM is widely employed, it is desirable to seamlessly integrate WDM or WDM-PON (passive optical network) transport systems with ROF systems.